Chemically, rhein is 9,10-dihydro-4,5-dihydroxy-9,10-dioxo-2-anthracene carboxylic acid having a structure of Formula I and diacerein is 4,5-bis(acetyloxy)9,10-dihydro-4,5-dihydroxy-9,10-dioxo-2-anthracenecarboxylic acid having a structure of Formula II. Diacerein is widely used in the treatment of osteoarthritis and has a unique mode of action that differentiates it from non-steroidal anti-inflammatory drugs (NSAIDs) and other conventional forms of drug therapy. Presently, diacerein capsules are available in 50 mg strength and are marketed by Negma in France under the trade name Art 50(R).

Diacerein is practically insoluble in solvents such as water, alcohols, acetone, dichloromethane and chloroform, which are generally used in pharmaceutical preparations. Although diacerein can be administered by oral route but it cannot be completely absorbed by the digestive tract, and this incomplete absorption results in undesirable side effects such as soft stools.
In order to overcome these problems, various derivatives, pharmaceutical compositions and specific galenic forms have been proposed in the literature. For example, European patent EP 243,968 describes a diacerein potassium salt, which is water-soluble and can be used in the preparation of compositions for parenteral administration.
Several patents/applications describe pharmaceutical compositions of diacerein. For example, EP243968 describes parenteral preparations of diacerein salts.
U.S. Pat. No. 6,124,358 and European Patent No EP904060 describe pharmaceutical compositions of rhein or diacerein, wherein rhein or diacerein is co-micronized with sodium lauryl sulfate. Although it is possible to improve the bioavailability of diacerein by co-micronization, it is still desirable to develop new formulations or new compositions which are likely to further improve the bioavailability.
U.S. Pat. No. 5,149,542 (EP263083B1); U.S. Pat. No. 4,861,599 (EP 264989B1) and U.S. Pat. No. 5,275,824 (EP 446753B1) describe controlled release or delayed release compositions.
U.S. Pat. No. 5,225,192 (EP 364944B1) and U.S. Pat. No. 5,569,469 describe different poorly soluble medicaments supported on polymer substances.
U.S. Pat. No. 5,952,383 and European Patent No EP 862423B1 provide pharmaceutical compositions of diacerein, rhein and their salts along with excipients.
There are several references known in the literature which disclose use of sugar alcohols like mannitol, sorbitol etc. as fillers in the formulations or as sensory cue agents i.e which impart feeling of cooling in mouth in case of orally disintegrating tablets (WO2007080601, EP589981B1, EP906089B1, EP1109534B1, U.S. Pat. No. 6,328,994, WO2007001086, US20070196494, US20060240101, WO2006057912, US20060057199).
In general, sugar alcohols like mannitol are employed in most orally disintegrating formulations and not in conventional immediate release formulations as sensory cue agents because the orally disintegrating tablets disintegrate in mouth instead of disintegrating in gastrointestinal tract as in the case of conventional immediate release tablets.
The present invention addresses and overcomes these commonly encountered problems of low solubility, incomplete absorption and soft stools.